2000 DAC/µP-Supervisor Combination Improves Analog Reliability
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2000
DAC/µP-Supervisor Combination Improves Analog Reliability
Using microprocessor supervisor, reliability analog improved systems employing industrial controller, programmable logic controller (PLC), data acquisition system. simple application shown implement this function. analog output zero when detects microprocessor failure undervoltage condition without need optocoupler. analog control signals used industrial controllers, programmable-logic controllers (PLCs), data-acquisition systems include 0-5V, 0-10V, ±5V, ±10V, 4-to-20mA current loops. safety reasons, some analog-output modules employ optocouplers electrically isolate microprocessor (µP) analog circuits. Also reliability safety, other systems require that analog outputs reset midscale zero response failure. These requirements circuit figure, which monitors activity sets analog output zero when detects failure undervoltage condition. also improves mean time before failure (MTBF) minimizing pins eliminating optocoupler.
When failure occurs supply voltage operation, this (U2) supervisor (U3) together produce user-programmed default output (VOUT). Three features enable (U2) produce zero-scale resets: asynchronous-reset input (CLR), user-selectable reset-value input (RSTVAL), user-programmable output (UPO) serial-interface setup. These built-in features also eliminate optocoupler, minimize pins lessen need external discrete circuitry. supervisor (U1) includes resistor-programmable reset-input threshold, capacitor-programmable watchdog timer, reset-timeout period. codes range from 000HEX FFFHEX, circuit output (VOUT) ranges from -10V +10V. offset DAC's internal, 10ppm/°C precision bandgap reference, output (U1B) configured buffer with gain four. Output voltage circuit VOUT VREF NB/4096 (RF/RIN where numeric value DAC's binary code, VREF internal reference voltage, gain U1A, RF/RIN U1B's gain-resistor ratio.
monitors isolated supply voltage (VISO) pin. also monitors activity pin, generates reset command RESET. resistor divider RESET (pin determines reset-level threshold (VTH). Similarly, reset timeout period (tRP) watchdog timeout period (tWD) determined capacitor values SWT. Connecting extends factor 500. following equations calculate values needed circuit: VRST VTH(R1 R2)/R2, where 1.22V VRST adjustable reset threshold voltage. CRST tRP/2.67, where CRST CSWT tWD/2.67, where fails, locks cannot toggle WDI, asserts output, which resets input. This action resets U2's output condition RSTVAL midscale this case, which produces VOUT). When being updated, output toggles U3's intervals less than nominal watchdog timeout (tWD seconds, subject tolerances capacitor ICs). Other devices consider watchdog function MAX6316 series MAX6369 series, which offer fixed timeout periods SOT23 packages. switch between U3's RESET terminals allows user command emergency shutdown. pulls high after reset timeout (tRP 60ms), output remains midscale until updated. DAC's unipolar bipolar modes allow produce common analog-output control signals.
similar version this article appeared November 2000 issue Electronic Design. D388, December 2000
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